In many personal and industrial applications, safety knives are desirable to prevent user laceration both before a cut is made and immediately subsequent to a cut. These safety knives may be disposable or designed for extended use. The knives that are used for extended periods may have replaceable blades or blade cartridges so that the handle and related safety apparatus may be reused while still maintaining a sharp cutting edge. The disposable knives may have the blade permanently attached to within the safety knife, so that the knife is discarded once the blade becomes dull. For example, the disposable knife is desirable in the food services industry, where loose blades from replaceable blade knives may find their way into the food product. A permanently attached blade forces the user to discard the entire knife rather than just change the blade. To reduce replacement costs incurred by the customer, these disposable knives are often made of inexpensive plastic materials with the least possible number of injection molded parts and moving parts.
Some safety knives have blade covers that may be retracted upon actuation of a trigger or similar actuation means. When locked, these blade covers are prevented from retracting due to the blade cover movement being blocked by a pawl-like mechanism that engages a stop. Only after being disengaged from the stop, can the blade cover be retracted to expose the blade. Some of these newer safety knives further include features that permit just one blade cover retraction for each trigger pull. If the trigger is continuously pressed after the blade cover has been retracted rather than released, the blade cover will still become locked in the extended position. Only after releasing the trigger and depressing the trigger a second time will the blade cover be permitted to retract once again.
The single-use per press feature has been achieved at this point through use of complex mechanisms with many separate parts. For example, much of the art has required the use of separate metal springs to bias parts towards locking. These separate springs and other parts increase the costs of manufacturing and reduce reliability.
Since many of the safety knives are disposable or otherwise have short lives, any added expense greatly detracts from the competiveness of the knife. It is desirable to have a safety knife that is inexpensive to manufacture and more reliable due to the reduction in parts and overall simplification of the mechanism.